1. Field of the Invention
The present invention relates to a document handling system and more particularly relates to method and apparatus for registering a document on a copier platen.
2. Prior Art
The present invention has particular utility in a copier wherein a document is placed on a platen and illuminated to cause an image of that document to be transmitted to a photoconductive member. The photoconductive member is charged to a uniform electric potential and receipt of light images from the platen dissipates that charge to form a latent image. Light areas or regions on the original dissipate the potential on the photoconductor. Dark or shaded regions of the original cause little or no discharge of the potential.
This latent image is subsequently developed using a powder which is attracted to those photoconductive portions which remain charged. As a result, dark portions of the original correspond to those regions on the photoconductive member which attract powder and light regions of the original attract no developer material. In a typical process the powder image is transmitted or transferred from the photoconductive member to a copy sheet which is then passed through a fuser station to permanently affix an image of the original to the copy sheet. This above-described process is commonly known in the art as xerography.
As the art of xerography has matured, improvements have allowed xerographic copying to be performed more efficiently and has afforded the user flexibility in how the copying is performed. Thus, high speed copiers can rapidly copy a number of original documents with information on both sides and produce collated, two-sided copy sets in a number chosen by the user.
Although high speed copiers can be operated in a manual mode, i.e. the document originals are manually positioned on a platen and individually illuminated, for high speed copying applications, the documents are more typically automatically positioned above the platen. In such high speed applications, a series of document originals are removed from a stack of such documents, driven along an input path to the platen, momentarily stopped on the platen in a registered orientation so that a document image may be formed, and driven from the platen for restacking in a stacking tray. For copying systems wherein the imaging optical systems have limited depth of field, it is also desirable that the original be flattened onto the platen to improve copy clarity. To rapidly yet accurately accomplish these steps, so called recirculating document handlers (RDH) are known. The functioning of one such recirculating document handler is disclosed in U.S. patent application to Sahay Ser. No. 071,613 entitled "Method & Apparatus for Copying Duplex Documents" now U.S. Pat. No. 4,278,344 which has been assigned to the assignee of the present invention. That application is incorporated herein by reference.
Many recirculating document handlers drive the documents onto the platen momentarily, stop document movement as the image is formed and drive them away from the platen without precisely side registering the document. Any side registration of the document that occurs is performed prior to moving the document onto the platen. In such systems, it is assumed that no misregistration or skewing of the document occurs as it is driven from its original position to the platen. This assumption, however, is not always justified with the result that skewed or misaligned copies can occur. If the problem of misaligned documents is to be addressed in a recirculating document environment, a mechanism for registering or positioning the document once it is on the platen is needed.
The prior art has dealt in piece-meal fashion with the various requirements discussed with respect to a high speed RDH operation. The prior art patents of which applicant is aware can be divided into two categories, patents which deal with positioning of the original with respect to the platen, and those patents which are concerned primarily with flattening the document original to ensure the image is within the optical system's depth of field.
The problem of accurately registering an original document on the platen has been recognized in the prior art. Various mechanisms have been utilized to corner register a document along orthogonal registration positions and in particular it is known to drive a document original against two edge registration guides using a rotating drive. U.S. Pat. No. 3,908,986, for example, shows coacting spherical and cylindrical drive members which drive a document into corner registration against two orthogonal guide members. The system disclosed in the '986 patent, however, is to be used with a hand fed copier system and it is questionable whether in a high speed copier it could accurately provide corner registration. U.S. Pat. No. 3,630,607 shows an alternative mechanism for corner registering a sheet. In that patent the cylindrical drive member of the '986 patent is replaced with a series of blades which intermittenty jog the document into a corner registration. Finally, U.S. Pat. No. 4,058,359 shows a drive roller which can be side shifted to properly position a document original along one dimension. The position of the document is first sensed and a control solenoid activated for the purpose of shifting the drive roller from side to side as required to maintain document alignment.
While the three patents discussed above recognize the need to carefully position the document or sheet along two dimensions, none of them addresses the problem of removing a registered document from a platen so that subsequent documents may be copied in order. The manner in which the document is driven away from the platen can be important. If the document is side registered against a registration guide, engagement of a drive mechanism without movement of the original away from the guide can result in damage to the original. There is no suggestion, for example, in the '359 patent to reverse the drive roller's position to remove the document from a registration position so that it may be driven from a platen.
Various patents are known in the art for clamping a document original against a platen to ensure the document is within the optical depth of field of the copier. For example, U.S. Pat. Nos. 3,623,806, 3,944,366, and 3,888,585 disclose various mechanisms for flattening a document original in relation to a platen. None of these references, however, show a high speed clamping mechanism which could be effectively utilized in a high speed document handler. Additionally, none of the references which relate to clamping discusses or discloses any mechanism for achieving a two-dimensional positioning of the document with respect to the platen.
To applicant's knowledge, no mechanism has been described which has the capability for corner registering a document with respect to a platen, followed by a clamping of the document to the platen for imaging which, in turn, is followed by side jogging of the document away from its corner registration position to allow it to be driven from the platen. As noted above, these steps must be performed rapidly and in synchronism and a study of the prior art references discussed above shows no such capability.